1. Field of the Invention
In general, the present invention relates to hydrogen diffusion cells and heating systems for hydrogen diffusion cells. More particularly, the present invention relates to heated hydrogen diffusion cells that contain palladium diffusion membranes.
2. Description of the Prior Art
In industry, there are many known techniques for separating hydrogen from more complex molecules in order to produce a supply of hydrogen gas. One such technique is electrolysis, wherein hydrogen gas is obtained from water. Regardless of how hydrogen gas is obtained, the collected hydrogen gas is typically contaminated with secondary gases, such as water vapor, hydrocarbons and the like. The types of contaminants in the collected hydrogen gas are dependent upon the technique used to generate the hydrogen gas.
Although contaminated hydrogen gas is useful for certain applications, many other applications require the use of pure hydrogen. As such, the contaminated hydrogen gas must be purified. One technique used to purify hydrogen is to pass the hydrogen through a hydrogen diffusion cell. A typical prior art hydrogen diffusion cell contains one or more palladium diffusion membranes. The palladium diffusion membranes can be palladium tubing or substrate supported layers of palladium. The palladium diffusion membrane is heated and the contaminated hydrogen gas is directed through the palladium diffusion membrane. When heated, the palladium diffusion membrane is permeable to hydrogen gas but not to the contaminants that may be mixed with the hydrogen gas. As such, nearly pure hydrogen passes through the palladium diffusion membrane and is collected for use.
Prior art hydrogen diffusion cells that use palladium diffusion membranes have problems associated with thermal stress. As a palladium diffusion membrane is repeatedly heated and cooled, it expands and contracts. The larger the palladium diffusion membrane is, the more it expands and contracts. As the palladium diffusion membrane expands and contacts, cracks may occur. Cracks are particularly prevalent at contact points where the palladium diffusion membrane contacts dissimilar material. Once a crack occurs in the palladium diffusion membrane, the hydrogen diffusion cell ceases to function properly.
In addition to thermal stress caused by normal operational heating and cooling, the palladium diffusion membrane in hydrogen diffusion cells are often subject to thermal shock. In the normal operation of a hydrogen diffusion cell, the hydrogen diffusion cell is heated to its operational temperature. Hydrogen gas is then introduced into the hydrogen diffusion cell. The incoming hydrogen gas is typically at a temperature that is much lower than the operational temperature of the hydrogen diffusion cell. The hydrogen gas rapidly chills the palladium diffusion membrane in the hydrogen diffusion cell, causing it to contract. However, the hydrogen gas heats rapidly and the palladium diffusion membrane quickly expand. This rapid contraction and expansion stresses the palladium diffusion membrane and is a root cause of failure of many hydrogen diffusion cells.
A need therefore exists for a system for regulating the temperature of a hydrogen diffusion cell so that thermal stresses experienced by the hydrogen diffusion cell are reduced. This need is met by the present invention as it is described and claimed below.